1. Field of the Invention
The present invention relates to a radio communication system, a radio communication control method, a radio communication apparatus, a control method for a radio communication apparatus, and a computer program that are used for performing radio data communication among terminals in a predetermined zone. More particularly, the invention relates to a radio communication system, a radio communication control method, a radio communication apparatus, a control method for a radio communication apparatus, and a computer program that are designed for a relatively small zone in a cellular radio or radio LAN applications.
Further particularly, the present invention relates to radio communication system, a radio communication control method, a radio communication apparatus, a control method for a radio communication apparatus, and a computer program that are intended to solve the problem in removing interference waves generated due to frequency repetition in an environment of a multi-cell formed of multiple micro-cells. The invention particularly relates to a radio communication system, a radio communication control method, a radio communication apparatus, a control method for a radio communication apparatus, and a computer program that achieve multi-cells and multiple accesses by carrying out fully autonomous distributed control.
2. Description of the Related Art
In recent years, there has been an accelerating demand for a broadband transmission also in the field of radio communication toward the coming broadband society. At the same time, there is an increasing demand for a radio communication system that permits higher efficiency of frequency use to use limited frequency resources more effectively thereby to efficiently accommodate an increasing number of subscribers.
To perform broadband radio communication, research and development efforts have been focused on a transmission method based on, for example, an orthogonal frequency division multiplexing (OFDM) modulation method. In the OFDM modulation method, which may be categorized as a multi-carrier transmission method, the frequencies of carriers are set such that the carriers are orthogonal with respect to each other in symbol sections.
However, the OFDM modulation method requires a wide frequency band, so that an adequate number of channels may not be secured in some cases. Such a case inconveniently prevents the use of a technique for removing co-channel interference attributable to frequency repetition.
For achieving improved efficiency of frequency use, a micro-cell cellular system having a smaller cellular radius is attracting the industry's attention as a promising replacement of a conventional macro-cell cellular system. The micro-cell cellular system allows the same frequency to be used more repeatedly with resultant higher efficiency of frequency use. It is, however, difficult to regularly arrange cells, and overlap of cells is likely to occur more frequently, making it difficult to regularly repeat the same frequencies.
Conventional cellular systems have been carrying out multiple accesses primarily by using the time division multiple access (TDMA)/time division duplex (TDD) method. In the TDMA communication method, the radio waves in the same frequency band in radio communication are assigned a plurality of time intervals or slots to enable a plurality of users to simultaneously use the frequency band. In the TDD communication method, an uplink and a downlink share the radio waves of the same frequency. Base stations operate in synchronization, and share standardized lengths and locations for uplinks and downlinks. This system, therefore, cannot successfully handle a case where a certain base station has more uplinks, while another base station has more downlinks.
In other words, the micro-cell cellular system fails to successfully remove the co-channel interference caused by the repetitious use of the same frequencies, or fails to permit free change of uplink/downlink ratios in some cases.